OVERCURRENT PROTECTION PICK UP GUIDELINES EXAMPLE

The following are recommended changes to the existing Over-Current Protection Pickup Guidelines:

1.       The minimum line to ground (LG) fault current will be calculated using a 10 Ohm fault impedance.
2.       The minimum phase fault current will be calculated using a 2 ohm fault impedance and will be the lesser of the phase to phase (LL) and a three phase (3p) fault. (2 ohms is the approximate arc impedance through air for our standard wire spacing)
3.       At a backup device (recloser or circuit breaker) we should strive (i.e. not mandatory) to detect a bolted fault at the end of the next device’s zone. This applies for both ground and phase settings.

Distribution Circuit Breaker
 Time Delay
 Phase Pickup
Minimum              = line ampacity or 2 x (maximum load current) which ever is the lowest
Maximum             = 1 phase (EOZ) 2 ohm (the lesser of the phase to phase and three phase fault, at the End OF ZONE, through a two Ohm impedance)
Max. Backup       = 1-LL (ENZ) (the bolted phase to phase fault, at the END of the Next Zone)

Ground Pickup

Minimum              = 0.3-0.5 x (maximum load current) or 0.3-0.5 x (line ampacity) which ever is the lowest
Maximum             = 1-LG (EOZ) 10 Ohm (the line to ground fault current, at the End Of Zone, through a ten Ohm impedance)
Max. Backup       = 1-LG (ENZ - the bolted line to ground fault, at the End of th Next Zone)

 Instantaneous
 Phase Pickup
Minimum              = 0.9 x 1-LL (EOZ)
Maximum             = 1.25 x 3phase bus fault

 Ground Pickup

Minimum              = 0.9 x 1-LG (EOZ)
Maximum             = 1.25 x 3phase bus fault

Recloser with Phase and Ground Settings
Phase Pickup
Minimum              = 2 x (maximum load current)
Maximum             = 1 phase (EOZ with 2 Ohm fault impedance)
Max. Backup       = 1-LL (ENZ)

Ground Pickup
Minimum              = 0.5 x (maximum load current)
Maximum             = 1-LG (EOZ with 10 Ohm fault impedance)
Max Backup        = 1-LG (ENZ)

Recloser with only Phase Settings
Phase Pickup
Minimum              = 2 x (maximum load current)
Maximum             = 1-LG (EOZ with 10 Ohm fault impedance)
Max. Backup       = 1-LG (ENZ)

The value of a (10) ten ohm ground fault impedance was initially chosen because it appeared to be the most prevalent in the industry from what could be determined. It is recognized that fault impedance from zero to forty Ohms are in use. The (10) ten ohm value gained legitimacy after reviewing EPRI Report EL-3085, Distribution Fault Current Analysis. This report indicates that 83% of faults involved the neutral or ground. The maximum fault impedance was calculated to be 3 Ohms.

As stated earlier, the value of a (2) two ohm phase fault impedance was chosen because it is the approximate arc impedance through air for our standard wire spacing.

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